The invention relates generally to a method and a apparatus for the direct regulation of output currents of an inverter, and more particularly to such an inverter feeding one or more induction machines without rotational speed sensors. The inverter uses a secondary current control to regulate the output currents. The secondary current control receives a current command variable system.
"Drehzahlveranderbare Antriebe in der Praxis" pages 34 to 38, Siemens, Bereich Energie- und Automatisierungstechnik, Order No. A 19100-E319-A365, discloses a pulse frequency converter which outputs a three-phase current system of variable voltage and frequency for a predetermined number of induction machines. The pulse frequency converter comprises an uncontrolled power rectifier, a dc voltage intermediate circuit and a machine-side inverter. The power-side current converter generates from the supply voltage a constant dc voltage which is smoothed by capacitors in the intermediate circuit. The machine-side current converter forms from the constant dc voltage, a three-phase voltage system with variable frequency and voltage. Transistors or gate turn-off thyristors (GTO thyristors) are used as current switches for the inverter. The pulse patterns for driving the inverter are generated in the microprocessor drive unit. For this purpose pulse patterns are selected which ensure optimal operation with nearly sinusoidal motor currents and minimum total losses, over the entire setting range of the frequency converter. The drive and regulation of the pulse frequency converter is fully digital. All functions including drive unit, ease of operation, and extensive protective functions are processed via a 16 bit microprocessor. A vector regulator is provided as the regulator, which achieves high regulating dynamics. The magnetization current and the active current forming the torque are determined from the available actual motor current and voltage. These current components of the current vector are compared with the setpoints. The difference is eliminated by regulation. In this way, given torque setpoints can be adhered to exactly. Rotational speed actual value sensors are not required up to a rotational speed setting range of 1:10.
Due to unavoidable switching and delay times of the switches of the inverter and the drive circuit whose durations are variable and not exactly known in the individual case (depending upon for example the parameter spread of the electronic components instantaneous current, and temperature) the frequency converter output voltage differs from the settings of the voltage drive unit. These erroneous voltage/time areas cause lower order harmonics and dc components in the output voltage, particularly with low frequency converter output frequencies. These harmonics lead to a non-sinusoidal curve of the output currents, to additional losses in the motor, and to concentricity properties which are not too good at low frequency converter output frequencies.
Marked improvement in the behavior of a drive is possible if the frequency converter outputs a regulated three-phase current system of variable frequency and amplitude, instead of a controlled voltage system. The non-ideal switching response of the power semiconductors as well as delay times occurring in the signal flow are detected by the regulator and eliminated by regulation.
DE 30 46 392 C2 discloses a method for controlling three-phase pulse inverters in which a three-phase sinusoidal current command variable system is given. The phase currents of an induction machine with a virtual neutral point following the pulse inverter are measured and compared to the command variable input. As a function of the deviation, the phases of the inverter are switched by means of hysteretic flip-flop controllers.
In groups drives wherein several motors, whose number can also be varied during operation, are connected in parallel, a field-oriented control cannot be employed. This results from the following facts:
(1) the different motors of the group drive can have different loads placed on the motors which cause the voltage drops across the winding resistances and across the leakage inductances to be different in each motor, which in each instance have a different phase position of the flux relative to the common inverter output voltage. If these motors are not equipped with a position detector the position of the flux cannot be determined. PA1 (2) the number of motors of the group drive is not known since the number changes during operation, also the nominal value for the flux-forming current components depending on the number or the type of the motors present is also hot known.
"Drehzahlveranderbare Antriebe in der Praxis" discloses that in group drives, frequency control is provided as the variable regulation, with the voltage given as a function of frequency and with current limiters. With single motor drives, a vector regulator as a rotational speed regulator without a rotational speed sensor but with current and moment limiters is provided as the variable regulator.
The present invention is directed towards the problem of developing a method and an apparatus for the direct regulations of invertor output currents by means of a secondary current control wherein this invertor feeds one or more induction machines without rotational speed sensors.